The role of blowdown is necessary to treat water in steam boiler. Blowdown is used to eliminate concentration of impurities and steam boiler should be free from scale. Amount of blowdown should be balance, do not too high and do not too low. If too high, the cost for chemical treatment and energy loss will high too, but if too low will make high concentration of impurities. There is not standard to regulate the amount of blowdown but practically it range between 1% until 25% regarding to the flow of feed water.
The flow of blowdown can be intermittent or continous flow depending on its use. In the classification of steam boiler based on tube type, there are two types of boiler, water tube steam boiler and fire tube boiler. In the water tube boiler, concentration of water in steam drum is controlled with continuous blowdown but in the mud/water drum or bottom header is controlled by intermittent to reduce possibility disruption of circulation in the steam boiler. In fire tube boiler, the flow of blowdown can be done with either intermittent or continuous. The amount, duration and frequency of blowdown can be controlled depend on requirement of water treatment, operating condition and design of steam boiler.